How To Stain Concrete with Copper Sulfate

Copper Sulfate Pentahydrate is sold as root killer.
You can buy a 2 pound bottle at a home improvement warehouse for about 8 dollars.
It will appear as either baby-blue power or brilliant blue crystals.
The pentahydrate is 36% water by weight but I imagine this is a lot cheaper per unit copper than buying the white anhydrous copper sulfate and adding your own water.

Ingredients

1/4 level cup Copper Sulfate (Root Killer)

1 cup warm water

I added the root killer to the water in a non-metal bowl and left it for two hours.
After this time, only about half of it was dissolved.
I put it into a large Pyrex pot and heated it gently on the stove.
With constant stirring it was all dissolved just about the time the first wisps of steam started to rise.
I wonder if this releases poison fumes?
The copper sulfate solution was clear and very bright blue.

Application

When I brushed the solution out over 10 square feet of gray concrete, it was so light that I was worried that the color would be too pale.
When it dried it was so light it was almost bluish white in some areas.

Results

After it dried, I wiped up the excess copper sulfate with a wet sponge.
There was much less excess powder than with the iron sulfate test patches.
So even though the color was pale, the stain definitely took to the concrete.
There were no visible brush strokes.
The color is much richer when the floor is wet.
If sealer were applied, the color would look about like this even when dry.

How To Stain Concrete with Copper Chloride

The only copper chloride I've seen for sale was lab grade ... too expensive for staining concrete.
So where to get copper chloride? Make it yourself from less expensive chemicals!

Ingredients

1/4 level cup Copper Sulfate (Root Killer)

1/4 cup (level) table salt

1 cup warm water

Combine ingredients and stir well until solids are dissolved forming a deep green solution of copper chloride and sodium sulfate.
You must use a plastic or glass container and stir with a plastic or wooden spoon because this recipe will discolor or even ferrous metal.

Application

I brushed the solution onto 10 square feet of gray concrete.
The liquid was a vivid green color.

Clean Up

The color changes as it dries.
It is no longer bright green, but rather a greenish blue "Statue of Liberty" color.
Sometimes it seems more blue and others greener.
I wiped up the excess with a wet sponge.

Results

The image below was produced by combining two photographs of the same spot.
Both photos were taken with the same camera.
The sunlight, angle, camera flash or the camera's automatic adjustment seems to alter the color.
This is an example of what I meant above when I wrote that it sometimes seems greener and others bluer.

Copper Sulfate + Calcium Chloride

On paper it looks like a good idea to make Copper Chloride from Copper Sulfate and Calcium Chloride because insoluble Calcium Sulfate (gypsum) precipitates out of solution.
It doesn't work out so well in practice because the gypsum is fluffy and doesn't just settle quickly to the bottom.
I tried to filter it out with a coffee filter but it just clogged the filter.
If I was more patient I guess I could have let it sit for a few days and then try to decant the liquid off top but I decided to instead move on to other approaches.

Copper Oxide + Hydrochloric Acid

Don't bother mixing basic copper oxide with hydrochloric acid to make cupric chloride.
Although the reaction is not very vigorous at room temperature, toxic chlorine gas is released.
Yuck!

Copper Carbonate + Hydrochloric Acid

Copper Carbonate is a much better ingredient for decorative concrete stain.
The reaction with acid has some significant bubbling and spitting, so you only want to mix a bit at a time and stir between additions.
The gas released from this reaction is regular old carbon dioxide... much nicer than the chlorine gas released by copper oxide.

I have recently done some experiments with this technique and will be posting details and photos soon.